etaloning flat

I'm doing I band photometry and am seeing etaloning in the images--I believe it's being caused by constructive and destructive interference due to the scraping of the chip and the gap between the chip and its mounting in the camera.

It's exactly the same pattern in each image, so I'm wondering if there's a way to compensate for it using a special type of flat (a normal flat won't get rid of it).  Does anyone know of an etaloning technique--a special type of flat or other solution--that will work?

Thanks.

Steve Smith (SSTB)